The present invention relates generally to bidirectional radio communication systems, and more particularly to bidirectional radio communication systems wherein one transceiver transmits frequency reference information to other transceivers, the radio communication system providing load control or utility use monitoring.
In a typical two-way radio communication system, a base station operating on one frequency transmits to a remote station, and the remote station transmits back to the base station on a related frequency. The relationship between the transmission frequencies of the base and remote stations is determined by the licensing rules of the Federal Communications Commission (FCC). For instance, in the Multiple Address System band the base station and remote frequencies are separated by 24 megahertz (MHz).
A dual carrier radio communication system becomes problematic when high through-put communications are required with a large number of stations spread over a large geographic area. One base station transmitter in communication with a number of remote stations will have a through-put determined by the bandwidth efficiency, as measured in bits per second per Hertz. High bandwidth efficiency transceivers are prohibitively expensive in systems which require many remote transceivers. In addition, if one powerful base station is transmitting over a large area there will be regions with poor reception, i.e. dead spots, due to geographic irregularities.
Alternatively, through-put may be increased by transmission of a plurality of carriers within an FCC approved band. This is termed frequency division multiplexing. The cost of this approach is usually in the increased frequency accuracy required of the radio transmitters. Frequency division multiplexing offers the additional advantage that the frequencies can be spatially reused; transmission regions (cells) which utilize the same pair of carrier frequencies are separated by cells which utilize different pairs of carrier frequencies, thereby minimizing interference. The through-put of such systems is equal to the product of the bandwidth efficiency, the number of cells in the system and the bandwidth of the carriers.
Accurate frequency control is conventionally accomplished by using quartz crystal resonators. With careful manufacturing techniques and control of temperature effects, an accuracy of a few parts-per-million is obtainable. Another standard frequency control technique utilizes feedback circuitry. For instance, a transceiver in conjunction with another transceiver with an accurate carrier can generate highly accurate signals utilizing two oscillators. First the signal is heterodyned to an intermediate frequency, then an accurate local oscillator at the intermediate frequency heterodynes the intermediate frequency signal to baseband, where the frequency and phase error of the intermediate frequency can be measured. This error is fed back to the first oscillator to correct its frequency.
With present technology, reduced frequency spacings can only be accomplished by using an outside source for a high stability frequency reference, such as WWV, GPS, or LORAN. The additional cost of including this refined capability in every radio within a system is prohibitive for many applications where low cost of two-way radio communication can bring substantial economic benefits.
An object of the present invention is to provide a high through-put bidirectional radio communications system between at least one base station and a large number of remote stations.
More particularly, an object of the present invention is to provide a low-cost method of generating radio signals at remote stations with a frequency accuracy necessary to provide frequency division multiplexing.
Another object of the present invention is to provide a low-cost method of generating a high accuracy transmission carrier at a remote station utilizing both information contained in the received carrier and information in the modulation of the carrier, particularly the clock rate of the signal.
Another object of the present invention is to provide a radio communications system wherein high data through-put is achieved at low cost by frequency synchronization, frequency division multiplexing, and time division multiplexing.
Another object of the present invention is to provide a receiver for decoding short data bursts from a plurality of transmitters.
Another object of the present invention is to simplify the circuitry of a receiver with clock rate and carrier frequency recovery.
More particularly, an object of this invention is to simplify the circuitry of a receiver with clock rate and carrier frequency recovery by providing a reception circuit which is coupled to the transmission circuit to produce transmission carrier stabilization, by providing independent clock rate and carrier frequency recovery from the received signal, and to provide a phase-lock loop at the baseband rather than the intermediate frequency level.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the claims.